The present invention in its various aspects relates to measurements of electric field, and measurements of electric potential, underwater from a vessel and calculation of the electric polarisation distribution characterizing that vessel.
The function of a magnetic range is to make measurements of the magnetic field from ships and submarines and provide information to set on-board countermeasures to minimize the threat from magnetic mines. A known transportable magnetic range is that sold under the trademark Transmag by Ultra Electronics Magnetic Division. A key part of the Transmag system is its ability to create a computer magnetic model of a ship. In other magnetic ranges, not involving modeling, the user sees magnetic measurements from sensors displayed on a computer screen, chart recorder or the like, and this data is as much affected by the ship passing the sensors at different distances and water depths and other environmental circumstances as it is by various changes which are made to the magnetic state of the ship. Consequently, the interpretation of the data is very difficult. By using a magnetic model in which a ship is considered as a distribution of dipoles, the measured magnetic model in which a ship is considered as a distribution of dipoles, the measured magnetic data is not shown directly but calculated data using the model is displayed. The displayed data is corrected so as to be as if the ship was passing over the exact cense of the range at a predetermined depth. Thus, the only variations in displayed data are due to real changes in the magnetic state of the ship. In consequence, interpretation of the data is made much easier but, more importantly, further processing by computer is possible.
Magnetic modelling has reduced the number of sensors necessary from dozens in some cases to a minimum of three if the ship can be sufficiently accurately navigated, or five to cope with a less competent navigational ability.
The magnetic modelling technique is described in a paper entitled xe2x80x9cMagnetisation Modelling Techniquesxe2x80x9d by Dr. G. J. Webb, The Royal Institution of Navel Architects International Conference Proceedings, 1994, Minewarfare, Vessels and Systems-3. The magnetic modelling technique is briefly summarised by the equation B=xcexc0D.m where B is the vector magnetic field at a point (this field has three vector components), m is the size of the vector magnetic moment which is the source of the field (three vector components), xcexc0 is the magnetic permeability of free space, and D is a matrix which contains the vector distance between the source moment and the field measurement point (a three by three matrix).
Ships and submarines each have an associated electric field; this is commonly called Underwater Electric Potential (UEP) field and is caused by corrosion between different metals in the hull and other underwater artifacts. For example, the propulsor is usually of an alloy, for example, phosphor bronze, whilst the hull is usually of steel. Some mines detect UEP using at least one pair of electric field sensors and detonate if the UEP is sufficiently large.
Whenever an electric field exists in a conducting medium, for example, the sea, there is an electric current. Whenever an electric current exists, there is around it a magnetic field. The corrosion current thus gives rise to a corrosion related magnetic (CRM) field. A problem with the corrosion related magnetic (CRM) field is that it decays more slowly with distance than that component of the magnetic field which is due to on-board magnetic items and degaussing coil currents and is known as the ferromagnetic field.
The degaussing of vessels is undertaken to protect them from magnetic field detecting mines by using on-board countermeasures, in particular degaussing coils, which provide a magnetic field equal and opposite to the vessel""s ferromagnetic field. In fact, the ferromagnetic field generally decays as the reciprocal of the cube of the distance whereas the CRM field only decays as the reciprocal of the square of the distance. This is illustrated in FIGS. 1 and 2, which are graphs showing relative magnitude of magnetic field against distance from the vessel in meters. In the both figures, the curve which is higher on the left hand side represents the ferromagnetic field, and the other curve is the CRM field. FIG. 1 shows the two fields from 1 to 100 m from the source. FIG. 2 shows the field at region around 200 m from the source. It is clear that, close to the source, the ferromagnetic field is much larger but, far enough away (in this case 200 m) the CRM field becomes larger.
The CRM field is not always negligible close to the vessel (where magnetic measurements are usually made). For certain types of ship it is possible that the CRM is 10% of the total measured field (CRM plus ferromagnetic field) or more if the ship""s degaussing coils are effective.
In its first aspect, the present invention relates to apparatus for determining an electric polarisation distribution of a vessel including a plurality of electric field sensors which are positioned in use underwater under the vessel, and processor means operative to process electric field measurements to determine the electric polarisation distribution of the vessel. The plurality of sensors are preferably arranged in a row, for example on the sea- or harbour bed. There are preferably between three and eight electric field sensors, and preferably five electric field sensors. The apparatus can be part of an electromagnetic range including a plurality of magnetic field sensors. The electric field sensors and magnetic field sensors are preferably arranged so as to be equidistant and in an alternating series along a row.
A corrosion related magnetic (CRM) field is preferably determined from the electric polarisation deduced for the vessel. Determination of CRM field can be useful in designing a vessel so as to minimise the risk of detection from the magnetic field, in particular CRM field, which it produces.
In its first aspect, the present invention also relates to a corresponding method of polarisation distribution determination from electric field measurements. Preferably corrosion related magnetic field is deduced from the polarisation distribution.
In its second aspect, the present invention also relates to apparatus for determining an electric polarisation distribution of a vessel including a plurality of electrical potential sensors which are positioned in use underwater under the vessel, and processor means operative to process electric field measurements to determine the electric polarization distribution of the vessel. The plurality of sensors are preferably arranged in a row, for example on the sea or harbour bed. There are preferably between three and eight electric potential sensors, and preferably five electric potential sensors. The apparatus can be part of an electromagnetic range including a plurality of magnetic field sensors. The electric potential sensors and magnetic field sensors are preferably arranged along a row such that each magnetic field sensor carries an electric potential sensor.
A corrosion related magnetic field is preferably determined from the electric polarisation distribution deduced for the vessel.
In its second aspect, the present invention also relates to a corresponding method of polarisation distribution determination from electric potential measurements. Preferably corrosion related magnetic field is deduced from the polarisation distribution.
Using electric potential sensors has a number of significant advantages. In particular, there is no requirement to align the electric potential sensors, so they can simply be fixed on top of a magnetic sensor. In a preferred embodiment, potential sensors are inherently less sensitive to positioning than electric field sensors. This is because the electric potential from a dipole source varies as the reciprocal of the distance squared whereas the electric field decays as the reciprocal of the distance cubed. Electric potential sensors are less sensitive to noise than electric field sensors. Electric potential sensors are typically cheaper than electric field sensors.
In its third aspect the present invention relates to apparatus for determination of the electric field of a vessel from its polarisation distribution. The present invention also relates to a corresponding method. This can be useful in designing vessels with low UEP fields, in particular so as to minimise the risk from UEP mines.
In its fourth aspect the present invention relates to apparatus for determination of corrosion related magnetic field from the polarisation distribution. The present invention also relates to a corresponding method.